Ethylene is a monomer that is used in preparing a number of olefin polymers. Ethylene is generally made by the pyrolysis or catalytic cracking of refinery gas, ethane, propane, butane, and the like. Ethylene, so produced, usually contains small quantities of acetylene. In polymer grade ethylene, it is generally preferred that the acetylene content be less than 5 parts per million by weight (ppmw).
One technique that has been used in the past for reducing the amount of acetylene in a stream containing ethylene is to selectively hydrogenate the acetylene using a catalyst comprising an active alumina carrier, a palladium component, and a silver component. In such a hydrogenation process, it is desirable for substantially all of the acetylene to be converted to other hydrocarbons, preferably ethylene, while only an insignificant amount of the ethylene is converted.
The selective hydrogenation of acetylene from a feed containing ethylene is promoted by the presence of carbon monoxide in the feed. Although a variety of palladium-silver catalysts have proven to be effective for selectively hydrogenating acetylene in feeds containing relatively high concentrations of carbon monoxide, many of these same catalysts are ineffective when the feed contains low concentrations of carbon monoxide (for example, less than 5 ppmw).
Because certain commercial hydrocarbon streams that contain low concentrations of carbon monoxide and high concentrations of ethylene also contain acetylene, there exists a need to develop a catalyst and process for selectively hydrogenating acetylene in such a feedstream.